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ATI's demos without vertex texturing
by Damien Triolet
Published on March 30, 2006

With the Radeon X1000, ATI missed a function that was apparently required to support Vertex Shader 3.0, the vertex texturing capacity which consists in accessing textures in vertex shaders (just like in pixel shaders). It’s important here to see textures in the broad sense, meaning as data tables and not only images.

How is it possible for ATI to support Vertex Shader 3.0 without vertex texturing? It’s apparently a trick, or to be more accurate, the exploitation of a loophole in DirectX 9 specifications. Vertex texturing support seems to come from the support of vertex shader 3.0 and there are no specifically devoted caps that indicate its support or absence. A GPU manufacturer, however, has to specify which level of filtering is supported in the vertex shader (point sampling, bilinear filtering, anisotropic filtering etc.) to support the vertex shader 3.0. Then for each texture format, it must be indicated if it’s usable in vertex shaders. And this is where the loophole is. Microsoft forgot to make at least one of the formats obligatory. In other words, ATI tells DirectX to support vertex texturing in point sampling but doesn’t authorize it on any texture format. It’s rather subtle, but enough to pass all DirectX specifications and not support vertex texturing in practice while obtaining Vertex Shader 3.0 certification.

ATI says it has vertex texturing support in point sampling mode, but doesn´t authorise it for any texture formats. Is this a clever way to bypass DirectX specifications?

Of course, this is a problem for developers who based their work on Vertex Shader 3.0 specifications and expected to find vertex texturing with ATI. That´s why NVIDIA has the advantage of having several additional graphic options in games like Pacific Fighters or Chronicles of Narnia.

Pacific Fighters with and without vertex texturing.

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